A post-processing apparatus is known which carries out a post-processing for a sheet conveyed from an image forming apparatus. The post-processing apparatus is provided with a sheet folding mechanism for folding a sheet bundle in half. The sheet folding mechanism comprises a first roller, a second roller and a folding member. The second roller is energized to the first roller. The second roller and the first roller form a nip section therebetween together. The folding member pushes a sheet bundle into the nip section. The second roller and the first roller fold the sheet bundle in half together through the nip section and then convey the sheet bundle folded in half to a discharging section. For example, the first roller is driven by a drive section provided with a motor to rotate. The second roller rotates with the rotation of the first roller.
Incidentally, in the sheet folding mechanism, it is needed to apply a pressing force with a magnitude by means of which a sheet bundle can be folded in half to the nip section. For example, as a method for applying a pressing force large enough to enable a sheet bundle to be folded in half to the nip section, a spring energizes the second roller towards the first roller. With the use of the spring energization, the pressing force of the nip section can be usually kept at a constant with which a sheet bundle can be folded in half. On the other hand, the pressing force of the nip section at the time of conveyance of a sheet bundle can be small as long as the pressing force is enough to convey the sheet bundle folded in half. However, if kept at a constant with which a sheet bundle can be folded in half, the pressing force of the nip section when the sheet bundle folded in half is conveyed may be excessively large. As a consequence, the sheet bundle folded in half may suffer a too large conveyance resistance when being conveyed, thus leading to problems such as the high power consumption of a motor.